1. Field of the Invention
The present invention generally relates to a method for synchronizing voltages of a fuel cell vehicle, more particularly, to a method for synchronizing voltages of a fuel cell vehicle, in which the voltages of high voltage components may be synchronized in real time regardless of the completion of vehicle start-up in order to improve performance and control precision within a full range of operable voltages.
2. Description of the Related Art
Generally, a fuel cell vehicle is driven by electricity generated by an electrochemical reaction of hydrogen, which is supplied as fuel, and oxygen in the air. A fuel cell vehicle includes not only a fuel cell stack for generating high voltage power from fuel but also various components for controlling output power, which are connected in parallel on a high voltage bus. Measuring devices or controllers for high voltage components include a stack voltage monitor (SVM), a motor control unit (MCU), a power conversion controller (a DC/DC controller), a blower/pump control unit (BPCU), a high voltage battery management system (BMS), and the like.
With regard to driving a motor when accelerating a vehicle and energy recovery through regenerative braking when decelerating the vehicle, the vehicle is controlled by systematic and cooperative control between high voltage components. Here, driving the motor and the output of regenerative braking may be controlled in such a way that a power conversion controller controls the voltage of a high voltage bus within a range from permissible minimum to maximum voltage thereof. In other words, when the voltage reaches the permissible maximum or minimum voltage while a vehicle is driven, each of the high voltage controllers performs output derating (limits the operation) in order to protect the high voltage components. Accordingly, when the voltages measured by all of the above-mentioned high voltage components have no difference, desired control precision and performance may be achieved.
For example, when a fuel cell control unit (FCU) transmits a voltage command for regenerative braking energy recovery to a power conversion controller, the power conversion controller controls a voltage depending on the command and stores the recovered energy in a high voltage battery. However, if the voltage differs from a voltage measured by a MCU, specifically, if the voltage measured by the MCU has a higher value, regenerative braking torque decreases due to the upper limit of the voltage, thus the desired amount of energy cannot be recovered.
Also, when a vehicle is about to slide backwards because it starts on an uphill slope after stopping, a motor rotates in reverse to prevent the vehicle from sliding backwards, thus some regenerative braking occurs. In this case, if the voltage of the MCU is greater than the voltage of the power conversion controller by a specific offset value, the voltage of the MCU increases by regenerative braking. As a result, when the voltage of the MCU reaches the maximum voltage in which regenerative braking is not allowed, derating is performed. When derating is performed in the MCU, the voltage decreases again and regenerative braking occurs. Consequently, because the regenerative braking, an increase in the voltage, and the derating attributable to the increase in the voltage are repeated, the vehicle may be subject to significant vibration. This phenomenon also occurs because the voltage of the power conversion controller depending on the voltage command transmitted from the fuel cell controller differs from the voltage of the MCU.
Also, when a stack generates power with low efficiency in order to increase the amount of heat from the stack during a cold start, the voltage of a high voltage bus is controlled depending on a minimum permissible voltage. In this case, the time for the cold start may be reduced only when the voltage is correctly controlled depending on the minimum permissible voltage under the condition in which the voltages of components have little error.
As described above, a power conversion controller controls the voltage of a bus depending on a voltage control command transmitted from a fuel cell controller within a range from the permissible minimum to maximum voltage. In this case, if the voltages, measured by the MCU, the power conversion controller, the BPCU, the high voltage battery management system, and the like, have errors, derating may occur. Specifically, during regenerative braking, derating may occur due to a component that first reaches the maximum voltage among the above-mentioned components. Also, when driving a motor or when generating power at low voltage during a cold start, derating may occur due to a component that first reaches the minimum voltage. As a result, it is difficult to achieve a targeted control performance of the fuel cell controller.
In contrast, when the voltages of all the high voltage components in the fuel cell vehicle ideally have the same value, abnormal operations and a control performance decrease caused by the difference of the voltages may be avoided.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.